U.S. Pat. No. 5,112,777 (MacDowell), assigned to the same assignee of the present application, discloses the use of glass-ceramic materials as binders for particulate ceramics, most specifically, alumina ceramics. Numerous uses for such composites were proposed in that patent. For example:
It was postulated that the oxidation of fibers entrained within ceramic bodies might be reduced through the presence of the residual glassy phase following the crystallization of the glass-ceramic bonding phase. It was also theorized that glass-ceramic bonded refractory phases, notably Al.sub.2 O.sub.3, cordierite, Si.sub.3 N.sub.4, SiC, TiN, and ZrO.sub.2, ought to allow lower sintering and hot pressing temperatures with increased mechanical strength and toughness, while continuing to maintain high use temperatures. Yet another proposal involved the inclusion of highly refractory particulate ceramics, such as Al.sub.2 O.sub.3 and/or ZrO.sub.2, as fillers in glass-ceramic coatings on the premise that those coatings would provide excellent barriers on various substrates to attack by oxygen and hydrogen. Finally, and the patent was principally directed to this utility, the useful life of abrasive products, with express reference to grinding wheels wherein particulate Al.sub.2 O.sub.3 constitutes the abrasive grain, could be extended through the use of a glass-ceramic, rather than a glass, bond.
The glass-ceramic materials disclosed in that patent contained a divalent metal borate crystal phase and were produced from precursor glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 25-65% B.sub.2 O.sub.3 and 20-75% RO, wherein RO consists of at least one divalent metal oxide selected from the group consisting of CaO, SrO, BaO, MnO, and ZnO, the sum of B.sub.2 O.sub.3 +RO constituting at least 55% of the total composition, with the following optional components in the indicated proportions of up to 20% SiO.sub.2, up to 15% F, up to 25% MgO, up to 10% ZrO.sub.2, and up to 40%. It was observed that, when Al.sub.2 O.sub.3 was present at levels greater than 20%, at least 2% F and/or 10% SiO.sub.2 would be included.